1. Field of the Invention
The present invention relates to a catalytic composite for purifying exhaust gases and a method for preparing the catalytic composite. More particularly, it relates to a catalytic composite capable of completely burning unburnt portions of hydrocarbons and carbon monoxide to decompose into carbon dioxide and water, the hydrocarbons and carbon monoxide being released from combustion equipment, automobiles, cooking utensils, and others using fuel such as petroleum, gasoline, or combustion gas.
2. Description of the Prior Art
As a catalytic composite for purifying exhaust gases, which converts hydrocarbon and carbon monoxide into carbon dioxide and steam in the coexistence of air, for example, a ceramic honeycomb structure is known, which is obtained by forming and calcining ceramic powder such as silica and alumina, and provided with a catalyst made of noble metals such as platinum, rhodium, or palladium. As the material for this ceramic honeycomb structure, cordierite composed mainly of alumina, silica, and magnesia is used from the viewpoint of durability. Since this cordierite is, however, high in density and is small in surface area, it is not suitable as the carrier of the catalyst. Therefore, to obtain a wide surface area, a coating layer with a wide surface area made of, for example, fine alumina particles is formed on the surface of cordierite. The noble metal catalyst is supported on the coating layer, resulting in a catalytic composite for purifying exhaust gases. Recently, perovskite-type compound oxides may be used as the catalyst instead of the noble metals. That is, on the surface of the ceramic honeycomb structure, the catalyst of perovskite-type compound oxides is supported together with an inorganic binder. In these examples, the catalyst is later supported on the surface of the ceramic honeycomb structure which was formed and processed.
In these catalytic composites for purifying the exhaust gases, however, since the catalyst is present only on the surface of the ceramic honeycomb structure, the area with the catalytic function is small. Besides, in cases where metal oxides are used as the catalyst, since the particle size thereof is larger as compared with the noble metal catalyst, the area with the catalytic function is further reduced. Therefore, when treating a large volume of exhaust gases, a sufficient catalytic activity cannot be obtained unless the load is reduced by increasing the size of the catalytic composite or the temperature of the catalytic composite is raised. Also, in the conventional ceramic honeycomb structure, the coating layer and catalyst are inferior in adhesion and hence they often peel off during use, lowering the catalytic performance. The lowering of adhesion may be improved by the use of an inorganic binder. When a large amount of inorganic binder is used, however, it may decrease the surface area of the coating layer; it may cover the catalyst surface; and it may react with the catalyst, thereby lowering the catalytic performance. When preparing these catalytic composites for purifying exhaust gases, three steps are required, that is, a step of forming and calcining a ceramic honeycomb structure, a step of forming a coating layer, and a step of allowing the catalyst to be supported. These steps are complicated and inferior in productivity, which may result in higher cost.